1. Field of the Invention
The present invention relates to a method of centering a constantly circular rim on a hub and the rotary device which corresponds to it.
2. Description of the Prior Art
Systems of storing kinetic energy lead to the development of rotors whose useful element is constituted by a rim driven at high peripheral speed in order to obtain a high kinetic energy per unit of mass.
Thus, rotors, in their present most developed form, are constituted by several functional elements:
the central portion or hub supported by magnetic suspension, and the magnetic circuit of the motor,
the rim which constitutes the largest part of the moment of inertia,
the linkages by arms between the central portion or hub and the rim,
the static and dynamic balancing means.
Several difficult problems however underlie the operation of such systems of storing kinetic energy.
The high centrifugal stresses in the rim which undergoes an elongation whose value exceeds the deformation limit occuring in current devices;
Certain constructions which may be considered thus lead to relative elongations of the rim which can reach 3% whilst the improvement in the breaking stress characteristics and in the continuous operation of composite materials even leads to the possibility of elongations reaching or exceeding 5%;
Rigid fastening of the rim or hub thus becomes almost impossible for several reasons connected with the fact that it is not firstly possible to contemplate carrying out this fastening by a bolt or other systems cutting the fibers of composite materials or destroying the homogeneity of the rim, for example of fused silica, and that subsequently the linking arms undergoing themselves considerable centrifugal stresses but different from that of the rim lead, for the same material and for a uniform cross-section, due to an elongation substantially one third of that of the rim whence a tendency to breakage under tensile stress in the arm-hub and arm-rim connecting zones and this, whatever the type of linkage concerned.
In the same way, the methods of fastening by "friction" between arm and rim contemplated in certain cases to absorb the expansion of the latter, cannot be retained on account of the impossibility of preserving the centering of the rotor in the course of rotation.
To this lack of suitable means for connecting the rim to the hub, must be added considerations of stability of the static and dynamic balancing, which amounts to maintaining, for all rotary speeds, coincidence between the axis of the rotor centering systems and the axis of inertia of said rotor; which coincidence must be kept stable despite the elongation of the rim of the rotor under centrifugal stresses as well as the variations in temperature and the cumulative effect of these parameters with the aging effect.
Lastly, the lift in the gravitational field of the rim whose mass can exceed 100 tons, requires a particular conformation of the rotor.
For certain applications, Applicant studied previous types of rotors with a sub-circular conformation in which the central portion of the rotor was provided with radial arms on which was wound, in a polygonal shape, a filamentary material constituting the rim.
On rotation, the rim takes up in these cases, under the effect of centrifugal forces, a shape comprised between a polygon and a circle, the resultant of the forces connected with the tension in the filament then always being directed towards the axis, resulting thus in compression of the arms.
If the shape of the rim were initially circular, the centrifugal force would introduce an elongation of the material involving a fastening on the arms, difficult or impossible to realize, which would make the latter work in extension, an effect adding to the elongation that they undergo themselves according to the previously explained system.
The optimization of such a configuration leads to an initial shape comprised between the polygonal shape and the limit of sub-circularity in order to still ensure compression on the arms.
Two rotors of this type were constructed in 1972 and tested up to 18,000 rpm; one was of steel wire wound on a hub with radial arms of light alloy, the other, of steel strip wound on the same type of hub.
Save for some limited applications, rotors with a sub-circular conformation present a certain number of drawbacks connected with the fact that the alternate flexions of the rim, at the rhythm of the variations in rotary speed, fatigue the material in the zones where it is supported on the radial arms and that it is difficult to position a device at the level of the rim, to cancel the static and dynamic unbalance.
Accordingly, it is an object of the present invention to provide a method of centering a circular rim on its hub, which does not present the aforesaid drawbacks and which offers in addition a practical solution to the problem of this static and dynamic balancing of rotors.